As you can see, i had to valve down the open channel too.
I would never do it without all three valves. Always better to have options IMO.
Whooooo...don't tell Uncle you feel that way!!
You do not need valves on the OC or emergency as these should never ever be restricted, at all. Ever. It's part of the core concept of the design. There is just no reason to have valves on these.
But why is having the valve at the sump level better? I don't see how that would make a difference. I couldn't do it anyway, though. I have an almost horizontal run right to the sump.
Definitely not ideal but was unavoidable in my case. And it's easier for me to make adjustments here rather than having to reach down under..
Because the concept of the siphon relies on the drain line being full. This means purging the air. So startup is the main reason to have the drain at the bottom. Once it's started and initially configured (first time you ever fire it up) then you tune the siphon valve such that you have 95% of the water flowing through there, or however it works best for you (usually a small amount of flow through the OC but not enough to close off the pipe and cause suction, but this is another subject).
Now, when you shut it off, let it settle out, then turn it back on, the siphon needs to purge out all the air. If the valve is at the bottom, the water crashing into the valve will close it off there, allowing any air in the line to get mixed up and pushed out rather quickly. If the valve is at the top, then this only occurs above the valve, meaning that it will take a lot longer to purge the air out of the line below the valve, if it ever does.
The horizontal run throws a big wrench in there though too, as I'm sure you are aware.
But all of this is irrelevant if your setup works for you. That is all that matters. Have you tested the failsafe operation by closing the siphon valve?
I tested things once by accident. I had a siphon hose snaked down the emergency drain so I could clean up some things and collect in a filter sock (great method by the way) but while doing this a big turbo snail went down the siphon line. Sounded like a break at a pool table. 3000 GPH still pumping. box filled up, OC kicked in, sucked box down...I had my hand on the pump power cord the whole time, I almost shat myself. Opened up siphon valve, out came snail, retuned...all good. Except I lost a few days off my life I'm sure.